Resonant cavity electrical transducer



April 24, 1962 M. H. DAZEY I 3,031,626

RESONANT CAVITY ELECTRICAL TRANSDUCER Filed Feb. 26, 1959 4 mt LeadMTCHELL ED/lZE v,

INVENTOR.

Patented Apr. 24-, 1962 3,031,626 RESQNANT CAVITY ELECTRICAL TRANSDUCERMitchell H. Dazey, 1120 Via Nogaies, Palos Verdes Estates, Calif. FiledFeb. 26, 1959, Ser. No. 795,765 4 Claims. (Cl. 331-67) The presentinvention relates to an electrical transducer, and more particularly toa transducer utilizing a resonant cavity into whch a minor perturbationis introduced for controlling the frequency of a close-coupledoscillator circuit of which the resonant cavity forms a principal part.

US. Patent #2,494,570 to Mezger issued January 17, 1950, discloses theuse of a resonant cavity in conjunction with an oscillator circuit toprovide a transducer. In the Mezger patent the oscillator circuit iscomplete without the resonant cavity. In other words, the oscillatorincludes a tank circuit of some type, and the resonant cavity merelyreceives a small portion of the electromagnetic energy from theoscillator without having any appreciable effect on the frequency orvoltage output of said oscillator. More specifically, according to theteaching of Mezger, the resonant frequency of the cavity and theoscillator frequency are intentionally established so as not tocoincide, and in fact, if the oscillator frequency does not remainconstant the expected potential changes due to small changes in theresonant frequency of the cavity will not occur as predicted.

While the Mezger device and other devices of a similar nature may havesome utility, they are also subject to Thus, a basic concept of thepresent invention is that the principle of the close-coupled oscillatormay be advantageously applied to a. resonant cavity transducer.

section, illustrating the principal mechanical construction features;and

FIGURE 3 is a partial cross-sectional View of the device illustratingthe complete electrical circuit connections in schematic form.

While the drawing illustrates a pair of transducer devices which aredifferentially connected to produce only a single output signal, it willnevertheless be understood that this is merely the preferred form of theinvention, and that a single transducer device may be utilized ifdesired. In referring to the drawing, letters and numerals used toidentify parts of one of the transducer devices will be primed toidentify corresponding parts of the other device.

Before describing the mechanical structure in detail it will beadvantageous to briefly outline in general terms the electricaloperation of one of the transducer devices in accordance with theinvention. An electrical resonant cavity A has a predetermined normalconfiguration. Means B are provided for introducing a controlled minorperturbation in the cavity, thereby to introduce a minor change in thenatural resonant frequency thereof. An electron discharge device C isprovided. which has at least three electrodes. Coupling means D areutilized to couple the cavity A with the discharge device C in such amanner as to provide an oscillator Whose oscillation frequency iscontrolled by cavity A and is substantially identical to the naturalresonant frequency ofcavity A. Means E is provided for tapping off aportion of the oscillating energy, in order to transmit same to afrequency indicating means.

For purposes herein a minor perturbation is defined as a change incavity configuration which is suiiiciently small so that the maximumchange in impedance relations, energy losses or frequency isof the orderof 1%. The changes will in general be much less than 1%, for example, if1000 megacycles is used as the operating frequency the maximum frequencychange may be as little Another feature of the invention is the use of apair of difierentially-connected resonant cavity transducer devices toprovide a more accurate measurement than could otherwise be obtained.

' One object of the invention is to provide a resonant cavity transducerhaving minimum expense and maximum utility.

Another object of the invention is to provide a resonant cavitytransducer comprising a pair of difierentially-connected transducerdevices from which a single output signal is derived. I

A further object of the invention is to provide a resonant cavitytransducer which utilizes minor perturbations of a vane whose angularposition may bevaried,'or a metallic plunger, or a plunger'of dielectricmateriaL'or a stream of gas of variable density and hence variabledielectric constant. phragm it is advantageous to use a tube F throughwhich a varying gas pressure may be injected into cavity A fordeflecting the diaphragm and correspondingly varying the resonantfrequency of the cavity.

The minor perturbation introduced by deflection of the diaphragm B, inthis example, serves to increase the natural resonant frequency ofcavity A and decrease the natural resonant frequency of cavity A', orvice versa. A difference frequency generating device suchas indicated byG is used to suppress the natural resonant frequencies of cavities A andA and generate only the difference frequency which varies linearly withdeflection of diaphragm B and the applied pressure.

The electrical operation of the transducer device hav- In conjunctionwith the flexible diaing been described in general terms, it now becomesconvenient to describe more specifically the mechanical structure of theillustrated form of the invention which, as previously, mentioned,includes a pair of differentially- 7 connected transducer devices.

Hollow metallic cylinders 10 and it) are provided on their open endswith outwardly extending circumferential flanges 11 and 11. A flexiblemetallic diaphragm B is transversely disposed between members it) andill and has its circumferential edge firmly grasped between flanges 11and 11'. Bolts 12a, 12b, etc. hold flanges 11 and 11' in tightly clampedrelationship.

In the remaining description the structure associated with cylindricalmember 10 will be described in detail,

and it Will be understood that a corresponding structure is associatedwith cylindrical member 18.

A hollow cylindrical metal core member 14 is concentrically disposedwithin cylindrical member iii. Cylindrical member ill at its closed endhas an inwardly extending flange 15 which is internally threaded toreceive a threaded base portion 16 of the core member 14. The annularspace between core member 14 and cylindrical member it? provides cavityA, While flange i5 and base portion 36 together provide a metallic endwall which closes off the space between cylinder and core, and consequently provides a short-circuited end of cavity A. The opposite orouter end of core member lid is closed ofif by a metallic wall 17 whichis physically spaced 8. short distance away from diaphragm B (for anypermissible degree of deflection of the diaphragm) so as to provide anopen-circuited end of the cavity.

Inwardly extending flange 15 has a threaded opening 20 provided thereinwhich receives a threaded tube F, the latter being utilized forinjecting a variable gas pressure into cavity A for controlling thedeflection of diaphragm B.

Base portion 16 of core member 14 has formed therein a threaded opening22 which communicates with the hollow interior of core member 14, and athreaded plug 23 upon which discharge device C is mounted is insertedinto opening 22 for the purpose of supporting the electron dischargedevice C within core member 14.

In the drawing, FIGURE 2 does not illustrate the electrical connections.These are shown in FEGURE 3, and it will be readily understood bypersonsskillcd in the art that the electrical details in FIGURE 3 are,for the most part, exaggerated for the purpose of clear illustration.

Electron discharge device C has at least three electrodes including aplate C a grid C and a cathode C Coupling means D includes anelectromagnetic coupling loop D; for supplying input energy into cavityA as well as a coupling loop D for receiving output energy from thecavity. A first connecting circuit D connects the.

plate C to loop D while a second connecting circuit 13.; connects loop Dwith cathode C The characteristics of the coupling elements D D are suchthat discharge device C and cavity A provide an oscillator whoseoscillation frequency is substantially identical to the characteristicresonant frequency of the cavity.

While coupling loops D and D are suitable for magnetic coupling to thecavity it will be understood that electrical probes may be substitutedfor either or both if so desired. Each of the coupling loops isinsulated from the metallic wall of the cavity at the point of entry bymeans of insulating bushings 2.5, in conventional fashion, and by-passedto ground by capacitors 35. grounded as far as radio frequency isconcerned by virtue of its connection to insulated plate 26 supportedfrom the inner wall surface of core member 14.

It is convenient to supply energizing potential to the oscillator byconnecting the otherwise free end of loop D to a suitable voltage (Bplus). Loop D and connecting circuit D consist of a hollow conductorwhose free end is accessible for external circuit connections, and whichcarries a filament lead 27 in its hollow interior.

The operating frequency is such that cavity A is some- I what less thanA wave length long. Connecting circuit Grid (3 is D is approximatelywave length long, adjusted for proper phase shift. In D and D, thefilament lead 27 is approximately A wave length long.

The energy tap E consists of a single conductor passing through abushing 25 in the wall of cylinder it? and hence through a rigid coaxialshield to a rigid housing 31 in which the difference frequencygenerating circuit G is contained. Circuit G preferably operates on theconventional thermionic diode or crystal rectifier principle.

Typical electrical operating characteristics which may be achieved areas follows. Utilizing a tube 6AF4 or 5703 together with a maximumvoltage of 300 volts and a grid drive of 20 volts, with a resonantfrequency of about 1000 megacycles, a loaded Q of about 8000 may beachieved with a sensitivity of 500 cycles per microinch deflection ofthe diaphragm, or equivalent endloading.

While a grounded-grid circuit has been illustrated, it will beunderstood that a grounded plate or grounded cathode could be used, orany other type of circuit con nection which would provide stableoscillation.

Where the transducer is utilized for position indication it isparticularly advantageous to use a plunger in lieu of the resilientdiaphragm as shown.

Many modifications of the invention will be apparent to those skilled inthe art, and it will be understood that I do not intend to be limited tothe details shown 01 described, except in accordance with the followingclaims.

I claim:

1. A transducer comprising, in combination: first and second rigidhollow metallic cylinders disposed in longitudinal alignment and havingabutting ends; a flexible metallic diaphragm disposed transverselybetween said cylinders and closing off said abutting ends thereof; firstand second rigid hollow metallic cylindrical core members concentricallydisposed in the outer ends of respective ones of said cylinders; firstand second rigid metallic plates closing off the outer ends of theannular spaces between said cylinders and said cores and rigidlysupporting the outer ends of said cores relative to the outer ends ofsaid cylinders; third and fourth rigid metal plates closing oil theinner ends of said cores, said coreinner ends being approximatelyequally spaced from the normal position of said diaphragm; first andsecond electron discharge devices housed within corresponding ones ofsaid core members, and each having at least three electrodes; first andsecond electrical circuit means respectively associated with said firstand second electron discharge. devices, each ofsaid circuit meanscoupling the associated discharge device to the cavity between thecorresponding cylinder and core for providing an oscillator whoseoscillation frequency is at all times substantially identical to thenatural resonant frequency of said cavity; mechanical means fordeflecting the central portion of said diaphragm along the longitudinalaxis of said cylinders so as to vary the resonant frequencies of both ofsaid cavities concurrently but in opposing directions; and meansfortransmitting oscillating'energy from both of said cavitiesconcurrently, to a difierence frequency generating device, so as toproduce a difference frequency indicative of the magnitude of thedeflection of said diaphragm.

V 2. A transducer as claimed in claim 1 in which each of said first andsecond plates has an opening therein adapted to admit gas to theassociated cavity, whereby deflection of said diaphragm is accomplishedby the differential gas pressures existing in said cavities.

3. A transducer as claimed in claim 1 in which each of said cylinders isprovided with an outwardly extending circumferential flange on its innerend, the outer circumferential portion of said diaphragm being disposedbe-- means pressing said flanges together for securely maintaining therelative positions of said cylinders and the outer circumferentialportion of said diaphragm.

4. A transducer as claimed in claim 3 in which said first and secondplates are partially provided by circumferential flanges extendinginwardly from the outer ends of said cylinders, and partially providedby circumferential flanges extending outwardly from the outer ends ofsaid cores, the mating circumferential surfaces of said flanges beingthreadedly engaged.

References Cited in the file of this patent UNITED STATES PATENTSLindenblad Dec. 16, 1941 Maeder June 21, 1949 Brown Nov. 29, 1949 KinzerMar. 14, 1950 Fiske et a1. Sept. 26, 1950 Kumpfer Oct. 17, 1950

